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We have designed new structures of carbon allotropes combining mixture of sp (triple bond) and sp 3 (single bond) hybridization through first-principle calculation. These two structures named Yne-carbon (Y-carbon) and Tetrayne-carbon (TY-carbon), respectively, not only maintained the cubic structure and space group (Fd3m) of diamond but were also energetically more stable than recently proposed carbon allotrope, T-carbon. A phonon calculation revealed these structures to be stable, and the nature of the triple bond was illustrated by the unique phonon spectrum with an eigen-frequency of 2200 cm-1. The band gap of Y-carbon was found to be larger than that of diamond, whereas the band gap of TY-carbon was smaller than that of T-carbon, which is closely related to the properties of a carbon tetrahedron. The existence of triple bonding of carbon is associated with a dimerization phenomenon due to Peierls instability.

Spiropyran (SP) is a mechanophore that undergoes changes in chemical structure and thus in optical character when subjected to a force. Incorporation of SP into polymer chains enables optical detection of stress/strain and damage in the material under load. However, the mechano-sensitivity of SPlinked polymers is in general insufficient for many applications. Attempts have been made to enhance the mechano-sensitivity of SP-linked polymers via modification of the chemical structure of the SP mechanophore or the mesostructure of the polymer matrix.In the present study, we explore how the mechano-sensitivity of SP incorporated in a poly(dimethylsiloxane) (PDMS) network is influenced by the pre-swelling of PDMS with an organic solvent (xylene). The effect of PDMS swelling on the optical property of the mechanophore was investigated by measuring the fluorescence intensity from the SP-linked PDMS in situ during uniaxial deformation using a custom-built opto-mechanical measurement setup. The results suggest that a longer swelling time causes a decrease in the initial fluorescence, a decrease in the activation onset strain, and an increase in the activation slope, resulting in enhanced apparent mechano-sensitivity. For instance, the 15 min pre-swollen SP-linked PDMS showed an 8.3-fold increase in mechano-sensitivity relative to the unswollen reference. Such effect is not limited to uniaxial extension but also observed in other modes of deformation, such as compression and bending. When corrected against the variation in sample thickness during uniaxial extension, and pre-strain and initial fluorescence intensity due to solvent swelling, the SP fluorescence intensity vs true strain (or chain entropy) curves obtained at various degrees of initial swelling collapse into a single curve, which suggests that the swellingenhanced mechano-sensitivity is due to the pre-strain of the network and a decrease in initial fluorescence intensity caused by the pre-swelling.

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The correlation between polymer architecture and molecular-level forces has long been a challenging research subject. Herein, spiropyran, a mechanophore that exhibits fluorescence change under force, was incorporated as a cross-linker between PMMA backbone segments. Using an in situ opto-mechanical setup to probe the molecular-level forces, the mechano-response of SP-linked PMMA as a function of the cross-link density was monitored during deformation. The dependence of the molecular-level force on cross-link density was quantitatively examined and revealed. First, a higher cross-link density shifted the fluorescence onset, that is, the onset of the spiropyran-tomerocyanine transition, to lower strains, eventually shifting the onset long before yield, without requiring sufficient chain mobility, owing to the higher efficiency of the force transfer. Under the same energy, the increase in cross-link density allowed for faster force transfer, but only to a certain level. Finally, the overall amount of spiropyran-to-merocyanine conversion linearly decreased with increasing cross-link density.